The invention relates to a screw extruder for processing thermoplastic materials, wherein a vacuum is applied to a feed opening for the material and/or to a filling section of a cylinder of the extruder for the purpose of reverse degasification, the extruder having a simultaneously rotating pressure piece located between the screw of the extruder and a back pressure bearing and a stationary end piece disposed on the screw side in front of the rotating pressure piece with a seal surrounding the pressure piece.
The seals, such as sealing rings, packing etc., used in known screw extruders are not adequate for ensuring the production and maintenance of a vacuum in the filling region of the screw extruder to effect reverse degasification.
Reverse degasification is frequently carried out during the processing of LD-PE. Usually an opening is provided on a part of the extruder cylinder on the gear side, and vacuum pumps are connected to this opening by way of vacuum lines in order to remove residual monomers such as ethylene gas from the extruder and thus to obtain a product free of gas occlusions.
It is further known to use retaining rings, which however only perform their sealing function in extruders with reverse degasification and to which a vacuum source is applied, up to a maximum pressure of 0.1 bar. When retaining rings are used it is not possible to apply a higher vacuum.